This is a multi-center, randomized, drug company initiated, placebo controlled trial of marimastat, a matrix metalloproteinase inhibitor (MMPI), in the treatment of glioblastoma multiforme or gliosarcoma inpatients who have completed surgery and radiation therapy.Multiple brain tumor centers will participate in this study where patients with receive marimastat (an inhibitor of tumor cell spread) or placebo (an inactive treatment) for their malignant glial brain tumors that have returned after surgery and radiation. Multiple brain tumor centers will participate in this study where patients with receive marimastat an inhibitor of tumor cell spread or placebo, an inactive treatment for their malignant glial brain tumors that have returned after surgery and radiation.